Climate Science Glossary

Term Lookup

Settings

Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup

Term:

Settings

Beginner Intermediate Advanced No DefinitionsDefinition Life:

All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 14 April 2017 by John Abraham

As humans put more heat-trapping gases into the atmosphere, like carbon dioxide, ice around the planet melts. This melting can be a problem, particularly if the melting ice starts its life on land. That’s because the melt water flows into the oceans, contributing to rising sea levels. Right now there are three main reasons that sea levels are rising. First, as ocean waters heat, they expand. Second, melting of ice in Antarctica flows into the ocean. Third, melting of ice on Greenland flows into the ocean. There is other melting, like mountain glaciers, but they are minor factors.

Okay, so how much is melting of Greenland contributing to sea level rise? Estimates are that about 270 gigatons of water per year are melting. The melting of an ice sheet like that atop Greenland can occur from the surface as air temperatures and sunlight warm the upper layer of ice. It can also occur from the edges as ice shelves collapse and fall into the oceans in large chunks.

Increase in surface melting from Greenland. Illustration: National Snow and Ice Data Center

For ice-shelf collapse, there’s a complex process that occurs at the bottom of the ice. Part of the ice is floating out over water and part of it is grounded on land. Warm water can get underneath the ice, lift it up, and melt the ice from below.

The bedrock underneath the ice sheet is not flat or gradually changing. There are undulations that rise and fall and change the water-ice-ground connection. Topology called “retrograde” can make it easier for ice to melt and can increase the rate of ice shelf collapse. So, scientists have a real interest in learning about the topology of the land underneath ice sheets so they can better predict ice collapse and sea level rise.

This brings us to a new study published by the American Geophysical Union in a journal called Geophysical Letters Review. The scientists use gravitometry to obtain a high-quality picture of the land underneath a very fast moving part of Greenland ice called the Jacobshavn Isbrae. Basically, the scientists flew gravity sensors across the ice at low altitudes and low velocity. These sensors are called accelerometers and they can be used to determine the x, y, and z gravity components. The measurements of the gravity allowed them to attain the local height of the subsurface with greater accuracy than previously known.

As stated in the paper, the motivation for this work was clear:

the fjord bathymetry and glacier bed topography of the lower portion of Jacobshavn Isbrae have remained poorly known. At least not sufficiently to provide reliable information for ice sheet numerical models.

They found that the trough underneath the ice was not symmetrically shaped; the northern part of the trough was deeper than the southern part.

Comments

Anyone studied the flood frequency in Greenland? The 2012 one in Kanger was well publicised but I think we are dealing with substantial increase here rather thana one in a century event like 2012 with unprecedented surface melt.

I think all infrastructure there maybe at risk of distuction by exponentially increasing flood frequency. So to those silly statements claiming some parts of the world would benefit from warming, in particular "Greenland will become greener" I would say: ain't necessarily so. "Greening" will not happen when country is about to be ravaged by serious floods. But I don't have any data confirming my fears over future of Greenland population. Obviously their culture - the disappearance of glaciers they admire and worship - is undisputable here. But on top of that their very existence maybe at stake. Will they adapt to more frequent flooding?

This piece commences "As humans put more heat-trapping gases into the atmosphere, like carbon dioxide, ice around the planet melts. This melting can be a problem, particularly if the melting ice starts its life on land." In view of this it is surprising that a recently published paper claims the Antarctic Peninsula is cooling. This paper (Oliva, M., et al., Recent regional climate cooling on the Antarctic Peninsula and associated impacts on the cryosphere, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.12.030) states "from 1998 onward, a turning point has been observed in the evolution of mean annual air temperatures across the Antarctic Peninsula region, changing from a warming to a cooling trend,". This is not an isolated observation as others (Carrasco, J.F. 2013. Decadal changes in the near-surface air temperature in the western side of the Antarctic Peninsula. Atmospheric and Climate Sciences 3: 275-281). and Turner, J., et al 2016. Absence of 21st century warming on Antarctic Peninsula consistent with natural variability. Nature 535: 18645.} have also shown a decline in Antarctic Peninsula temperatures. Warming in the Antarctic has been claimed to be due to atmospheric C02 but it now seems that CO2 may in fact cause cooling in the Antarctic Peninsula (Holger Schmithusen, Justus Notholt, Gerd Konig-Langlo, Peter Lemke, Thomas Jung. How increasing CO2 leads to an increased negative greenhouse effect in Antarctica. Geophysical Research Letters, in press, 2015. doi: 10.1002/2015GL066749.)

1) The OP is about Greenland, not Antarctica, so discussion of the Antarctic Peninsula is technically of topic.

2) This is Figure 1 from the first paper you reference with regard to Antartic Peninsular temperatures (Oliva et al):

You will firstly note that 7 out of 10 temperature records are from the northern tip of the Peninsula, or islands lying still further north; and that consequently a simple mean of the temperature records does not show temperature trends over the peninsular as a whole due to spatial bias. Second, you will note that since 1998 temperature trends for the northern 7 stations and the 3 stations representing the bulk of areal extent of the peninsula have had opposite signs. That means (at best) of three regions of approximately equal area (northern, south west, and south east), we have one region warming, one cooling, and no data for the third. On that data, claims of cooling for the entire peninsular go far in excess of what is supported by the data. We can reasonably claim the pensinsula as a whole warmed from the 1950s to about 2000 as all records, and the records for the two regions with data showed that trend (and the trends in the cryosphere in the third supported it); but cannot make a claim to peninsula wide cooling since then if we are guided by the data.

3) Here is Figure 5 from Schmithusen et al:

You will note in particular that even with 4 x CO2, there is a positive change in regional forcing over the entire Arctic (though weakest over Greenland). You will further note that the regions of a negative change in the regional greenhouse effect are strictly limited to parts of East Antarctica, far from the sea. If you compare this with Fig 4 you will see that (approximately) Greenland forcing increases from a current mean value of about 5 W/m^2, as also does the West Antarctic Peninsula. East Antarctica, in constrast gets a stronger negative greenhouse effect relative to that which already exists there (on annual averages).

Further, despite the current negative regional forcing in central East Antarctica, temperatures there are still well above that which would be expected from insolation alone. That is because warm air comes in from the north. With ongoing warming in the north, the temperature gradient between maritime air above the circumpolar waters and that in the interior of Antarctica will increase, resulting in a stronger heat transfer between the two. Over the areas with an increase in radiative forcing (such as the Antarctic Peninsula), that will result in more warming than expected from regional changes in radiative forcing alone.

It is unlikely that Greenland itself will suffer from sea level rise. In addition to the hydrostatic rebound, both instantaneous and long term, as the ice decreases, it's gravitational pull which pulls the surrounding water toward it like hitching up a skirt, will decrease. Incidentally, this release of the skirt of water will raise sea level even more in other locations than just due to the flow of Greenland glacial water into the ocean.

On a different topic there should be a dynamic process increasing the melt of the ice sheets where they are grounded below sea level. As the ever warming ocean water melts the ice front, the water in contact with the ice becomes less salty and hence lighter. This water should flow up under the ice ceiling, pulling in more warm ocean water along the sea bottom. The deeper the grounding line the greater this effect should be so the process should accelerate where the sea bottom is on a retrograde slope.

Tom at 3: Thanks for that excellent summary of Antarctica dynamics, I enjoyed it and learned a lot. However, Haze is off topic and, frankly, seems to just be trying to create haze, not to provide a valid comment on Greenland's potential catastrophe. His comment should simply be removed, in my opinion.

I am of the, admittedly badly informed, opinion that Greenlanders will be fine, and benefit once they adapt. The rest of us would not be so lucky.

I fear the isostatic adjustment is much more serious than we realize; volcanism throughout the world may be its dichotomous side-effect, especially if at the same time East Antarctica rocks the continent as mass shifts from West to East. Volcanic and tectonic pressure points in other parts of the geode may slip catastrophically adding to CO2 loads, while reducing uptake, from temporary sudden cooling from vocanism.

PluviAL @5, still futher off topic, but the relocation of water from ice sheets near the poles to oceanic water near the equator will also change the duration of the Earth's day. The change in angular momentum involved will not be uniformly transferred to the interior, providing another trigger for earthquakes, and potentially vulcanism. Whether that mechanism, or the two you mention will raise either earthquake or volcanic frequency appreciably above background rates, however, is SFAIK, unknown.

I agree that my comment does not relate directly to Greenland and as such is, strictly speaking, off topic. However I am not trying to create a haze but to generate discussion and perhaps through that gain a greater understanding of why Greenland and the Antarctic Penisular react differently to global warming. If this discussion is not relevant then no doubt the moderator will heed the exhortation of PluvIAL at 5 to remove my comment. I would note however that the dscussions at Skeptical Science are becoming increasingly constrained, and thus decreasingly informative, due to the current relative paucity of comments on most topics

I am surprised that you feel "discussions at Skeptical Science are becoming increasingly constrained, and thus decreasingly informative, due to the current relative paucity of comments on most topics" (unquote)

My own impression (admittedly personal anecdotal) over the past 4 years of frequent observation of SkS, is that there is a good flow of informative discussion in the comments columns. If anything, I feel there is an even higher amount of such discussions than 3 or 4 years ago. On top of that, I must say (from reading many comments columns back to the previous decade) that I noticed fluctuations in activity in most of the lengthier columns — but that is only something to be expected, as new developments and new data come along from time to time. Perhaps, Haze, you are interested in a narrow range of climate topics : and that may have caused you to have a certain bias in your observations.

Possibly the most important factor in discussions is the absolute need (if any!) for any discussions in the comments column! A good many of the articles posted are so straightforward, that little discussion is required — save for the rebuttal of erroneous or peevish disputations that may appear in the comments column.

00

Moderator Response:

[DB] Thank you for your fine response, but as the comment you were responding to was moderated, it became necessary to also moderate your comment's response to the off-topic material.

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.